Welsh Borderland - Malvern Hills

From Earthwise
Jump to navigation Jump to search
The Malvern Hills, looking north from near Wynds Point, comprising hard igneous rocks of the Malvern Complex. P213697.

The Malvern Hills area forms a north−south running ridge extending between the Central area and the Worcester-Great Malvern area (Plate P213697). This ridge was uplifted in ancient times by movement on the Malvern Fault.

Sedimentary bedrock[edit]

The youngest rocks in the Malvern Hills area are located to the south of Ledbury. These rocks are more extensively seen at depth within the Worcester-Great Malvern area to the east of the Malvern Fault. The bedrock geology at the surface comprises a sequence of soft, brick-red sandstones of the Sherwood Sandstone that are about 200 m thick. These are underlain by brown, red or grey sandstones which contain mudstone layers and pebble seams and are present at surface in a small area around Bromesberrow Heath, probably extending up to 700 m below the surface around Newent. These two sandstone layers together are 300 to 250 million years old. Underlying them is a sequence of Warwickshire Group comprising red and grey mudstone, sandstone, coal beds and the occasional thin layers of limestone. These rocks have not been exploited for coal in this area. This sequence was deposited approximately 320 million years ago and has a proven thickness of over 350 m in boreholes near Newent. In places within the Malvern Hills it is likely to reach to depths of at least 1000 m. These layers are tilted towards the south and rest upon the much older rocks that are observed at the surface in the northern part of this region.

Basement rocks[edit]

To the western side of the Malvern Fault the 5–km–long Malvern Hills run north to south underlain by rocks that range in age from 1000 to 415 million years old. In the northern parts of the Malvern Hills the influence of the fault makes ascertaining the thickness and depth of the various rock layers uncertain. These rocks are tilted towards the west and so when traced eastwards from Ledbury to near Hollybush they get progressively older toward the Malvern Fault.

The youngest rocks in this sequence are the same red-brown Raglan Mudstone (Silurian) that is seen across most of the Central part of the region. This unit is at the surface near Ledbury but thins from a maximum thickness of about 400 m in the west to nothing to the east. Underlying the red-brown mudstone are the same dark grey Silurian mudstones described previously in the Central area. This rock layer likely extends to depths of about 700 m near Ledbury. Underlying this are the well preserved fossil-rich limestones which are likely to be at depths of about 900 m below the surface; below this are the 440 million year old green-grey mudstones. These are likely to reach depths of over 1200 m based on their thickness observed elsewhere. Below this and reaching unknown depths is a sequence of grey to green finely layered mudstones. The oldest basement rocks, called the Malvern Complex, are visible at the surface north of Hollybush at Midsummer Hill.

In addition directly west of the Malvern Fault a number of slivers of hard, dark grey intrusive rocks formed from molten magma rising from very great depths are present at surface. These rocks are likely to be over 500 m wide and probably extend to depths of 2000 m.